Question: Multiply the following complex numbers, marked as blue dots on the graph: $(7 e^{\pi i / 2}) \cdot ( e^{7\pi i / 4})$ (Your current answer will be plotted in orange.)
Answer: Multiplying complex numbers in polar forms can be done by multiplying the lengths and adding the angles. The first number ( $7 e^{\pi i / 2}$ ) has angle $\frac{1}{2}\pi$ and radius $7$ The second number ( $ e^{7\pi i / 4}$ ) has angle $\frac{7}{4}\pi$ and radius $1$ The radius of the result will be $7 \cdot 1$ , which is $7$ The sum of the angles is $\frac{1}{2}\pi + \frac{7}{4}\pi = \frac{9}{4}\pi$ The angle $\frac{9}{4}\pi$ is more than $2 \pi$ . A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $\frac{9}{4}\pi - 2 \pi = \frac{1}{4}\pi$ The radius of the result is $7$ and the angle of the result is $\frac{1}{4}\pi$.